1. Field of the Invention
The present invention relates to a semiconductor light emitting device that uses a III-V nitride-based compound semiconductor, and a manufacturing method thereof.
2. Description of the Background Art
In recent years, research and development of semiconductor lasers where an AlxGayIn1-x-yN (x≧0, y≧0, x+y=1) III-V nitride-based compound semiconductor quantum confined structure is used as an active layer have been actively conducted for semiconductor light emitting devices which can emit light ranging from blue to ultraviolet rays that is required in order to increase the density of optical disks, and some have already used in practice. These are primarily fabricated by processing a wafer that includes a nitride-based compound semiconductor layer where a crystal of a III-V nitride-based compound semiconductor has been grown on a sapphire substrate.
It is known that, in an AlxGayIn1-x-yN (x≧0, y≧0, x+y=1) III-V nitride-based compound semiconductor of which the crystal has been grown on a sapphire substrate via a so-called “low temperature buffer layer”, highly dense penetration dislocations of about 109/cm2 are formed, due to the difference in the lattice constant between sapphire and AlxGayIn1-x-yN.
This density corresponds to the existence of a penetration dislocation in a square region of which one side is almost 1 μm. It is considered that such a penetration dislocation becomes the center of non-radiative recombination where electrons and holes which thermally move are captured, so that they are made to disappear during the process of non-radiative recombination.
Accordingly, it is considered that in order to achieve high efficiency in light emission, space regions where the difference in the energy level between electrons and holes is relatively small may be locally formed with high density, so that electrons and holes may be prevented from being captured in the center of non-radiative recombination such as penetration dislocation.
In fact, it has become clear, through experimentation using cathode luminescence measurements (see, for example, S. Chichibu et al., “Exciton localization in InGaN quantum well devices”, J. Vac. Sci. Technol., 1998, B16(4), p2204–2214), near field optical microscope measurements (see, for example, A. Kaneta et al., “Discrimination of local radiative and non-radiative recombination processes in an InGaN/GaN single-quantum-well structure by a time-resolved multimode scanning near-field optical microscopy”, “Applied Physics Letters”, 27 Oct. 2003, Volume 83, Number 17, p. 3462–3464) and the like, that a high In region having a size on a nanometer scale (so-called “quantum disc” or “quantum dot”) is formed in a well layer in a quantum well structure where the well layer is made of Inx1Ga1-x1N and the barrier layer is made of Inx2Ga1-x2N (x1>x2≧0).
It is considered that macroscopically, the reason why such a high In region is formed is based on the difference in the solid solubility between In and Ga in an InGaN mixed crystal. In addition, it is considered that microscopically, the cause is based on the substantial difference in the atomic size between In and Ga, such that stability of energy in the system is achieved by the integration of In at the temperature where the crystal is grown.
The size of high In region and the difference in the composition vis-à-vis the surroundings in such an InGaN mixed crystal can be controlled to a certain extent by adjusting a variety of growth conditions such as the growth temperature at the time of crystal growth, the V/III ratio, the amount of flow of hydrogen, and the growth rate. However, the bonding energy differs to a great extent between In—N and Ga—N; therefore, it is difficult to make the distribution of In in the crystal uniform, in comparison with AlGaInAsP-based mixed crystals, which are typical compound in the conventional art.
As described above, in the case where an AlxGayIn1-x-yN-based compound semiconductor is used, in particular, Inx1Ga1-x1N is used for the quantum well layer, a high In composition region is locally formed in a natural manner, and this constrains electrons and holes and prevents electrons and holes from moving into penetration dislocations. As a result, a light emitting device using a nitride semiconductor crystal that has been grown on a sapphire substrate has relatively excellent device properties, in spite of the fact that the density of penetration dislocations in the crystal is about 109/cm2, which is extremely high.
The optical properties of a crystal for a quantum confined structure, such as a quantum well structure, as described above, can be evaluated by the peak intensity of emitted light in the photoluminescence properties, primarily at room temperature. Semiconductor light emitting devices which are widely used are assumed to be utilized at room temperature; therefore, good photoluminescence properties at room temperature become the determination standard for evaluating the optical properties of the quantum confined structure.
In the case where the bonding energy of excitons is ignored, photoluminescence measurement is one of the most common optical evaluation methods in compound semiconductors where the structure that becomes the object of measurement is irradiated with a laser beam having a wavelength that is shorter than the wavelength that corresponds to the energy for the quantum confinement in the structure, that is, the difference in the energy level between electrons and holes and the light that is radiated from this structure is separated into a spectrum so that the intensity thereof can be measured.
Strictly speaking, the physical light emitting process is different between an actual semiconductor light emitting device and the light emitting process in the photoluminescence measurement. That is, in a semiconductor light emitting device, a current is injected and, thereby, electrons and holes are introduced into the quantum confined structure. On the other hand, in the photoluminescence measurement, the quantum confined structure is irradiated with a laser beam and, thereby, electrons and holes are generated.
In addition, in a semiconductor laser device having an extremely high electron and hole density in the quantum confined structure, Coulomb interaction which works between electrons and holes is blocked by electron-electron scattering, and this can be ignored. However, in the photoluminescence measurement, the intensity of the conventionally radiated laser beam is relatively faint; therefore, the density of electrons and holes which are generated in the quantum confined structure is small, and excitons are formed between electrons and holes due to Coulomb interaction, and after that, radiative recombination occurs.
As described above, physically, there is a slight difference between the light development mechanism in an actual semiconductor light emitting device and the light emission mechanism in the photoluminescence measurement. However, the photoluminescence measurement is frequently used as a technique for directly and simply evaluating the optical properties of the quantum confined structure, and can be said to be the most common optical evaluation technique.
In fact, in the case of a device of which the intensity of outputted light is relatively faint, it is considered that the stronger the photoluminescence intensity is at room temperature, the better efficiency of light emission is obtained in a semiconductor light emitting device that includes a quantum confined structure of which the crystal has been grown on a sapphire substrate.
As described above, in the InGaN quantum well layer having high photoluminescence intensity at room temperature of which the crystal has been grown on a sapphire substrate, spatial change in the In composition ratio suppresses the movement of carriers within the plane of the well layer. As a result, non-radiative recombination in defects such as penetration dislocations are prevented; thus, relatively excellent device properties are exhibited.
A spatial change (fluctuation) in this In composition ratio within the plane of the active layer, however, simultaneously causes spatial non-uniformity in the difference in the confining energy between electrons and holes in the InGaN quantum well layer; therefore, the effective volume of the semiconductor light emitting region that can contribute to light emission is reduced in a semiconductor light emitting device for outputting light having a specific wavelength. As a result, this becomes optical a factor in deteriorating the device properties, such as efficiency of light emission.